Method of effecting pre-stress



Oct. 12, 1965 A. HASSELLOF 3,210,837

METHOD OF EFFEC'IING PRE-STRESS Filed Dec. 2, 1963 1N ENTOR 44% 6255543;

United States Patent 3,210,837 H METHOD OF EFFECTING PRE-STRESS AlkeHassellof, Surahammar, Sweden, assignor to Allmanna Svenska ElcktriskaAktie'bolaget, Vasteras, Sweden, :1 Swedish corporation Filed Dec. 2,1963, Ser. No. 327,236 Claims priority, application Sweden, Dec. 5,1962, 13,081/ 62 2 Claims. (Cl. 29404) The present invention relates toa method of effecting pre-stress or increasing the pre-stress incomposed constructions and to pre-streissed constructions manufacturedor adapted to increased pre-stress in accordance with this method.

In order to effect pre-stressed constructions a pressure means isnecessary, which with support against the construction in question ischarged with great force, which is thereby transmitted to theconstruction and compresses its parts so that the desired pre-stress isobtained. The pre-stress is effected thus by tightening or stretchingthe pressure means at erection until the parts of the construction arecompressed with sufiiciently great force. This procedure is suitable insuch cases where use is made of simple pressure means such as tensionrods, draw bolts or the like. The method is on the other hand detailedand difficult to execute in such cases where other pressure means areused, for example strip or thread casings wound around the construction.

The method according to the present invention is especially suitable forsuch cases, since the construction may be at first built up withoutpre-stress or with very little pre-stress. Further, the method should beused to pre-stress or to obtain an increased pre-stress in composedconstructions, which are normally subject to an intermittent workingload, such as for example press frames, rolling stands and motor frames,whereby according to the invention the construction is loaded with aforce being at least equal to the desired pre-stress and adapted tocharge the construction in a way statically imitating said normalworking load, so that a clearance occurs between at least two of theparts of the composed construction, after which filling pieces areinserted in said clearance.

Especially for such pre-stressed frames which include two substantiallysemi-circular yokes separated by two pillars and held together by aprestressed thread or strip casing wound around the yokes and pillars,the method according to the invention leads to a considerablesimplification of the manufacture. Such frames have namely beenmanufactured previously by winding a steel strip or the like under heavydrawing pressure in several layers around pillars and yokes but thisknown procedure necessitates an expensive apparatus and is both detailedand costly. After the winding of the strip casing has been carried outthe pre-stress can no longer be regulated, but according to the methodaccording to the invention the pre-stress in such a frame may easily beincreased to a higher value if for some reason it is too low.

The invention will be described in more detail in the following withreference to the accompanying figures which show schematically how themethod is adapted to a frame of the type described above.

FIGURE 1 shows a frame including two semi-circular yokes 1 and 2separated by two pillars 3 and 4 and held 3,210,837 Patented Oct. 12,1965 together by a strip casing 5 wound around yokes and pillars. Theframe in FIGURE 1 is erected without any real pre-stress and in order toobtain pre-stress in the frame, which is necessary if the frame shall beable to take up forces without clearance arising between its parts, theframe is charged with two forces P and P parallel with the pillars andworking towards the yokes. These forces which are equal and directed inopposite directions are symbolised in FIGURE 2 by two arrows. The forcesP and P should be at least equal to the desired pre-stress in the frame,but preferably considerably greater in order that the desired pre-stresscan with certainty be attained. When the non-stressed frame according toFIGURE 1 is charged by the above-named forces the strip casing 5 isstretched and a clearance 6 occurs between the upper yoke 1 and thepillars 3 and 4. In these clearances one or several filling pieces 7 areplaced, which retain the frame in the extended position according toFIGURE 2 even after the forces P and P have been removed. The pressurein the strip casing 5 then also remains substantially unchanged and thedesired pre-stress has thereby been obtained.

The method according to the invention may be adapted to all kinds ofprestressed, composed constructions irrespective of which form thepressure means has. The adaptation of the method is thus not limited tothe shown type with pressure means in the form of a thread or stripcasing but several variations are feasible within the scope of thefollowing claims.

I claim:

1. Method of effecting a desired degree of pre-stress in a constructioncomprising a plurality of pillars separating a pair of yokes at oppositeends of a surrounding strip casing, said method including the steps ofapplying equal forces in opposite directions to said yokes to stretchsaid casing and give it a stress at least equal to the desideredpre-stress, thereby providing clearances between said yokes and the endof said pillars, then inserting fillers to substantially fill each ofsaid clearances and removing said forces, said fillers thereby retainingthe construction in extended pre-stressed condition.

2. Method of producing a pre-stressed frame construction comprising apair of yokes separated by a plurality of intermediate pillars and aclamping means surrounding said yokes and pillars, said method includingthe steps of fitting said yokes and pillars loosely in said surroundingclamping means, applying equal forces in opposite directions to saidyokes to stretch said clamping means and give it a stress at least equalto the desired final pre-stress, thereby providing clearance betweensaid yokes and the end of said pillars, then inserting fillers tosubstantially fill each of said clearances, and removing said forces,said fillers thereby retaining the construction in extended pre-stressedcondition.

References Cited by the Examiner UNITED STATES PATENTS 1,872,115 8/32Byerlein 29452 XR 2,539,056 1/51 Brown 29-447 2,938,260 5/60 Holben etal. 29l56.4 3,115,696 12/63 Evans 29149.5

WHITMORE A. WILTZ, Primary Examiner.

THOMAS H. EAGER, Examiner.

1. METHOD OF EFFECTING A DESIRED DEGREE OF PRE-STRESS IN A CONSTRUCTIONCOMPRISING A PLURALITY OF PILLARS SEPARATING A PAIR OF YOKES AT OPPOSITEENDS OF A SURROUNDING STRIP CASING, SAID METHOD INCLUDING THE STEPS OFAPPLYING EQUAL FORCES IN OPPOSITE DIRECTIONS TO SAID YOKES TO STRETCHSAID CASING AND GIVE IT A STRESS AT LEAST EQUAL TO THE DESIDEREDPRE-STRESS, THEREBY PROVIDING CLEARANCES BETWEEN SAID YOKES AND THE ENDOF SAID PILLARS, THEN INSERTING FILLERS TO SUBSTANTIALLY FILL EACH OFSAID CLEARANCES AND REMOVING SAID FORCES, SAID FILLERS THEREBY RETAININGTHE CONSTRUCTION IS EXTENDED PRE-STRESSED CONDITION.